Hyperthermia treatment, in particular, is based on the greater sensitivity of tumour cells to temperatures above 41° C. compared to healthy cells. Magnetically mediated hyperthermia is based on the generation of heat by magnetic nanoparticles through exposure of the latter to an oscillating magnetic field. Compared to other methods of hyperthermia, magnetically mediated hyperthermia is one of the less invasive approaches that is more promising in biomedicine, since magnetic nanoparticles can offer a number of advantages: i) the nanometric size of the nanoparticles would allow intravenous injection and transport via the bloodstream to reach tumours that cannot be reached otherwise; ii) the high surface/volume ratio permits functionalization of the surface of the nanoparticles with one or more recognition molecules, ensuring orientation towards specific tumoral tissues; iii) remote heating of the magnetic nanoparticles by applying an external magnetic field limits the heating action just to the zone where the nanoparticles accumulate, minimizing the side-effects of heating.
The heating capacity of the magnetic nanoparticles exposed to an alternating magnetic field is expressed by the specific absorption rate (SAR), which provides a measure of the absorption of energy per unit of mass of the magnetic material when it is exposed to electromagnetic waves. The generation of heat is derived either from hysteresis losses or relaxation processes (Néel or Brown). The SAR values depend on the structure, composition and crystalline quality of the nanoparticles, but also on the frequency (f) and the amplitude of the magnetic field (H) applied during the measurements. For efficient thermal treatment with minimum invasiveness for the patient, it is of fundamental importance to have magnetic nanomaterials that show high SAR values at a low dose of magnetic nanoparticles and at a low frequency and/or amplitude of the magnetic field applied. In this connection, it has been observed experimentally that there is a biological limitation for the amplitude of the applied magnetic field (H) and the frequency (f). Various studies have demonstrated that the product of the amplitude of the magnetic field and of the frequency (H·f) must not exceed the threshold value of 5×109 A m−1 s−1 for the hyperthermia treatment to be considered safe for the human body. Many of the SAR values of magnetic nanoparticles reported in the literature are measured at frequencies between 500 and 700 kHz and fields between 10 and 20 kAm−1, with consequent factors H·f that, in most cases, exceed the threshold value. Moreover, the lack of standard devices or of established measurement protocols contributes to increasing the variability of the SAR values measured up to now.
In recent years, attention has been focused on obtaining superparamagnetic nanoparticles smaller than 12-14 nm, and many protocols or treatments provided are based on particles of this size. Hergt et al. (Journal of Magnetism and Magnetic Materials 2005, 293, 80) presumed that high SAR values could be reached in the range of dimensional transition between superparamagnetic nanoparticles and ferromagnetic nanoparticles, which in the case of iron oxide nanocrystals (IONCs) is approx. 20 nm. Moreover, to obtain “injectable” nanoprobes, superparamagnetic nanoparticles should be preferred to ferromagnetic ones: absence of residual magnetization of these nanoparticles in the absence of the applied external magnetic field allows better dispersion and avoids the problems of aggregation that are typical of ferromagnetic materials.
At present, the available superparamagnetic nanoparticles have low SAR values. Moreover, a dramatic decrease in thermal power of superparamagnetic nanoparticles has been observed, once they are transported into cells or tissues. To overcome this limitation and to achieve a reasonable increase in temperature, studies were carried out in vitro in which higher doses and frequencies were used, but exceeding the safety limits by a factor H·f.
Among the various materials that have shown promising magnetic properties (high saturation magnetization, relatively high anisotropy constant Keff and a high initial magnetic susceptibility), iron oxide nanocrystals are by far the most studied, also owing to their biocompatibility and availability. These nanocrystals can in fact be prepared in large amounts with simple methods, such as sol-gel techniques or co-precipitation techniques, and have already been tested in preliminary clinical trials of hyperthermia.
It has been demonstrated, however, that nanocrystals prepared by these methods have the drawbacks described above (low SAR values in solution and a further decrease of these values when the particles have been or were localized to the biological tissue).
Moreover, the nanocrystals obtained with sol-gel or co-precipitation techniques exhibit high polydispersity, which in theory would compromise the heating power with a decrease in SAR value. In contrast, particles synthesized by methods of thermal decomposition have shown excellent magnetic properties, but even in these cases the magnetic behaviour is highly dependent on the method of synthesis employed and so is rather unreliable.
Detailed procedures for synthesis of colloidal ferrite nanocrystals have been amply described. In general, most of these procedures are based on the decomposition of precursor species in a solution containing various stabilizers (preferably organic surfactants) in particular reaction conditions. The species form the so-called “monomeric species”, which then react, giving rise to nucleation and a growth phase of the nanocrystals. Organic stabilizers are fundamental for slowing the growth process, so that it can be controlled and for keeping the size of the crystals at the nanometric level. Moreover, the stabilizers, effectively coating the surface of the nanocrystals, prevent these from forming aggregates during growth. In some cases they can contribute to control of the shape of the nanocrystals.
The procedures described above make it possible to obtain almost monodisperse colloidal nanocrystals up to 15 nm in size. New methods have recently been reported with which it is possible to obtain iron oxide nanocrystals larger than 15 nm using “seed-mediated growth” synthesis, “one pot” synthesis, or chemical transformation of iron oxide of other phases. SAR values have recently been reported for sugar-coated iron oxide nanocrystals with size between 4 and 35 nm, synthesized with various colloidal methods. In particular, those prepared by thermal transformation of annealing of FeO nanoparticles to Fe2O3 nanoparticles showed the highest performance in hyperthermia. A recent work describes iron oxide nanocrystals ranging in size from 6 to 18 nm obtained by refining a “seed-mediated growth” technique with control of the polydispersity of the nanocrystals that is below 2%. However, these particles have low SAR values. In a more detailed magnetic and structural study, it was found that low hyperthermia performance may be a consequence of their magnetic core/shell structure, consisting of a magnetic core, with size corresponding to that of the starting seed, enveloped in a magnetically frustrated layer. The study suggested that this “seed-mediated growth” method might not be the most suitable if good hyperthermia performance is desired.
Various methods have been proposed in recent years for the synthesis of ferrite nanoparticles with increased control of shape. In these cases too, however, uniformity of shape is limited to just some size ranges of nanoparticles. Guardia P. et al. recently reported, in Chemical Communications 2010, 46, 6108, a synthesis for producing iron oxide nanocubes in a wide range of sizes. The method is able to overcome the aforementioned limits, reaching dimensions of nanoparticles up to 180 nm.
In the proposed technique, iron(III) acetylacetonate, decanoic acid and dibenzyl ether were mixed together and heated. Particle size can be controlled using an appropriate ratio of iron precursor to decanoic acid or by following suitable heating ramps. Even if the magnetic behaviour of these nanocubes makes them suitable for various applications, the proposed synthesis nevertheless has various drawbacks, such as non-uniformity of shape (in fact obtaining irregular cubes, spheres and multi-faceted particles together with the regular cubes), and a wide distribution of particle size of about 20% and up to 30%, which has a negative influence on hyperthermia performance. Moreover, the synthesis is poorly reproducible and the effective size of the final nanocrystals cannot be predicted. Finally, the nanocrystals obtained by the synthesis are soluble in non-polar solvents, thus making it difficult to use them in aqueous environments typical of biological systems.